1. Field of the Invention
This invention relates to a method for producing a detecting element to be used for measuring the flow rate of fluid by utilizing a dependence on temperature of the resistance value of the element.
2. Related Art Statement
There has hitherto been known, as the detecting element of this kind, a detecting element, comprising a planar or cylindrical substrate consisting of glass, ceramics or metal, and an electrically conductive member consisting of thin film, thick film, wire or the like, and arranged on the substrate (in the case where metal is used as the substrate, the electrically conductive member is arranged on the metal substrate through an insulating material). A metal lead is fixed to the substrate by an electroconductive paste or glass and connects the electrically conductive member and an external circuit.
The electrically conductive member has been subjected to a baking treatment (heat treatment) by means of a heating furnace (electric furnace) at the formation of a protecting layer which is formed in order to protect the electrically conductive member from the external atmosphere.
However, in the above described conventional method, wherein the heat treatment is carried out by means of a heating furnace, glass is generally used as a protecting layer. Therefore, in the conventional method, it is necessary to use a noble metal, such as platinum or the like, as a metal substrate or as a metal lead in order to prevent the metal lead from being oxidized during the baking of the glass, and hence the resulting detecting element is expensive.
Alternatively, a base metal lead may be used in place of a noble metal lead. However, in this case, it is necessary that a heat treatment is carried out under a reducing atmosphere, N.sub.2 atmosphere or Ar atmosphere in order to prevent the oxidation of the base metal lead.
However, glass to be used for a protecting layer is easily foamed or decomposed during the heating under a reducing atmosphere, N.sub.2 atmosphere or the like, and due to this foaming phenomenon and the like of glass, the resistance value and the resistance temperature coefficient value of the electrically conductive member are varied. Accordingly, in order to use a base metal lead, it is necessary to control strictly the production condition and the material (glass or the like).